Micromembrane actuator

ABSTRACT

Thin film shape memory alloy (SMA) micro pump that has high work density and high frequency response. A miniature SMA pump is used to rectify liquid to achieve stroke. Controlled fluid flow provides forced convection cooling on the SMA membrane that allows the micro pump to operate at high operational frequencies and high work density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a compact actuator that utilizes small thin shape memory alloy (SMA) diaphragms to provide large force output at high drive frequency. The actuation is based on a shape memory alloy miniature pump, which rectifies liquid to achieve stroke for the actuator. A bias pressure is applied to bend the SMA membrane upward to form a cavity between the membrane and a surface in the actuator body upon which the unbiased membrane sits. A pulse of charge is used as resistive heating so that membrane accomplishes a plunging stroke towards the surface when heated, thereby forcing liquid out of the cavity. High drive frequency is reached by impinging the liquid on the heated membrane to achieve forced convective cooling. The heated liquid flows out via the outlet port. Adding additional membranes in parallel also increases the flow rate.

2. Description of Related Art

The publications and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the references are numerically referenced and grouped in the appended bibliography.

Thin film SMA possesses unique characteristics that are attractive for use in actuators. A foremost of those characteristics is a large strain output, which can typically strain up to 8-10%. No other active materials posses this behavior, and studies have shown that fatigue life can exceed million cycles when strains are below 2% [1]. It also has the highest work densities for smart materials, for instance 25×10⁶ joule/m³ for NiTi compared to 0.1×10⁶ joule/m³ for piezoelectric materials [1]. A less well-known attribute is the thin film's high frequency response due to increased heat dissipation from large surface-to-volume ratio. While bulk SMA typically has frequency responses of less than 1 Hz, thin film SMA can have frequency response on the order of 100 Hz if power delivered to the thin film is carefully manipulated to account for heat transfer issues [2]. These attributes make thin film SMA an attractive material for micro actuation devices.

Typical micro devices require large deformations while exerting sufficient forces from an actuating material. This work density is an inherent attribute of thin film SMA and has been used in SMA based micropumps. One of the first micropumps, the pumping mechanism was based on two antagonistic 3 μm thick NiTi membranes [3-6]. For this pump, the push-pull pumping motions were generated by alternately heating the membranes. The pressure head generated by this motion was 519 Pa while operating at drive frequency of 1 Hz. At higher drive frequencies, the membranes were not sufficiently cooled thereby reducing the pumping motion and the flow rate. The maximum flow rate was 50 μl/min. Therefore, this pump did not produce large flow rates or large force outputs due to the limitations of the system.

Makino and his colleagues also developed a SMA-based micropump but used a single NiTi diaphragm biased with pressurized nitrogen gas [7-10]. Their pump was able to operate at a drive frequency of 0.2 Hz, which was sufficient for cooling and shape recovery of the diaphragm. Separate studies also revealed that 6 μm thick NiTi diaphragms displayed larger force outputs under 500 kPa bias pressure. The flow rate was 4.8 μl/min, which was achieved under bias pressure of 100 kPa. This pump lacked the qualities of large force and large flow rates.

More recent development in thin film SMA micropump was based on a bimorph design where strips of 5 μm thick NiTi were adhered on top of 15 μm thick silicon membrane [11-14]. With this design, a 100 Hz drive frequency was reported, but they also pointed out that insufficient cooling subsequently reduced diaphragm stroke causing reductions in flow rate so that this drive frequency was not sustainable. The maximum flow rate was 350 μl/min at drive frequency of 60 Hz.

SUMMARY OF THE INVENTION

The present invention provides a thin film SMA micro pump actuator that has high work density and high frequency response. The invention uses a miniature SMA pump to rectify liquid to achieve stroke. The invention manipulates the fluid flow to have forced convection cooling on the SMA membrane, eliminating the insufficient cooling of prior art designs. The result is an improved design with operation at flow rates well in excess of prior art designs.

Past micropumps have not exploited the critical properties of thin film SMA which are high work density and high frequency response. The actuators of the present invention are able to achieve high output force at large velocities by exploiting both high frequency response and work density properties of thin film NiTi SMA membranes. The actuator uses a miniature SMA pump to rectify liquid to achieve stroke, which past micropumps did not consider. The insufficient cooling associated with past micropumps is also eliminated. This is achieved by manipulating the fluid flow to have forced convection cooling on the SMA membrane. By doing so, the flow rate for the current actuator increases to three times the order of magnitude higher than the past micropumps.

Additional features of the inventions are provided in the following detailed description of the preferred embodiments with reference to the drawings.

The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing assemblage of a single membrane pump actuator with SMA membrane and a retaining lid.

FIG. 2 is a cross-sectional view taken along the planes 1A-1A and 2A-2A shown in FIG. 1.

FIG. 3 is a cross-sectional view of a deformed SMA membrane under bias pressure.

FIG. 4 is a front view of a pumping chamber.

FIG. 5 is perspective view of the four-membrane pump actuator formed accordance with present invention.

FIG. 6 is a perspective view of a SMA membrane that is suitable for use in pump actuators in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A pump-based compact actuator is provided in accordance with the present invention that is capable of producing a large force output and a large volume flow rate. This following detailed description sets forth exemplary embodiments that are a few of the many considered possible for this actuation system and as such this description is regarded as an example.

The actuators of the present invention utilize at least one thin shape memory alloy membrane for means of actuation. Multiple membranes may be used to increase volume flow rate. The force output of the pump can be controlled by varying the properties of the membrane, such as the thickness of the membrane. The actuator includes an actuator body that includes surfaces that define one or more actuation chambers that hold SMA membranes over a membrane seat surface that includes a liquid inlet port and multiple outlet ports. These ports are located relative to each other so that cool liquid impinges on the hot membrane for faster heat transfer through forced convection cooling. The heated liquid then flows out through an outlet port in the actuator or pump body.

The SMA membrane is heated by electrical resistive means by passing a pulse of charge through the SMA membrane. Each current pulse heats the SMA membrane thereby causing the membrane to accomplish plunging stroke, which pushes the liquid out of the chamber. The flow rate is dictated by repetition of current pulses.

In the following description, numerous details are set forth in order to provide a more thorough description of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well known features have not been described in detail so as not to unnecessarily obscure the present invention.

A perspective view of an embodiment of the present invention is illustrated in FIG. 1. The actuator comprises a pumping chamber or actuator body 2 a, 0-ring 4 that is preferably made from TEFLON®, inlet porthole 5, outlet portholes 6, inlet 7, and retaining lid 1 with SMA membrane 3. The actuator body includes surfaces that define and actuation chamber in which the membrane is located. The bottom surface of the actuation chamber is a membrane seat surface as shown at 25. The 0-ring 4 provides a seal between the lid 1 and the actuator body 2 a. The retaining lid 1 is pressed on to the actuator body 2 a such that junctions between the TEFLON® 0-ring 4 and pump chamber 2 a and TEFLON® 0-ring 4 and SMA membrane 3 are watertight. A small cavity 9 is formed between the membrane seat surface 25 and the SMA membrane 3. The retaining lid 1 and pump chamber 2 a are electrically insulated.

The membrane seat surface 25 is preferably dome-shaped and has the inlet 5 located in at the top of the dome and the outlets 6 located equidistantly around the outer perimeter of the domed surface. The inlet 7 in the actuator body is joined to inlet 5 to provide fluid flow into the actuation chamber. The fluid introduced through inlet 5 is at a temperature that is below the martensite-austenite transition temperature of the given membrane material and it is introduced at a bias force or pressure that is sufficient to move the membrane from its undistorted form (adjacent to the domed seat) to a distorted form where the membrane is displaced away from the domed-seat 25. Upon heating to a temperature above its martensite-austenite transition temperature, the membrane seeks to return to its original undistorted shape. This exerts the necessary force or pressure to move the fluid in the cavity between the membrane 3 and the domed-seat 25 out of the actuation chamber through outlets 6.

The membrane 3 may be viewed as having an active side that is adjacent to the domed-seat 25 and an inactive side. The membrane 3 divides the actuation chamber into a pump chamber 9 (see FIG. 2) located between the membrane 3 and the domed-seat surface 25 and an idle chamber located between the inactive side of the membrane 3 and the lid 1. The volume of the pump chamber increases (and the volume of the idle chamber decreases) when the membrane is moved from it undistorted form to its distorted form upon application of bias pressure when the membrane is below its martensite-austenite transition temperature. Upon heating of the distorted membrane, the membrane moves back to its undistorted form with sufficient force to overcome the bias force and move liquid out of the pump chamber. Relatively cool liquid (at least below the martensite-austenite transition temperature for the membrane) is again introduced under bias pressure to cool the membrane so that it can be distorted again once the membrane falls below its martensite-austenite transition temperature. The flow of fluid provided by the inlet-outlet configuration in the domed seat surface provides for effective convective cooling of the membrane to allow more rapid pumping cycles.

In order to prevent fluid from exiting the inlet 5 during high pressure operation (movement of heated membrane from its distorted form to its undistorted form) a flow control mechanism, such as a check valve (not shown) is provided insure one-way fluid flow at that inlet port 5 into the pump chamber. An outlet port 10 (FIG. 4) is connected to the outlets 6 by an appropriate manifold to provide removal the fluid from the pump chamber and actuator body 2 a. The outlet port 10 also includes some type of flow control mechanism to insure that fluid does not flow back into the pump chamber when the membrane is under the lower bias pressure or force. Exemplary outlet flow control mechanisms may include a check valve (not shown) to prevent reverse fluid flow from the outlet back into the pump chamber.

Referring now to FIG. 2, cool liquid (at least below the martensite-austenite transition temperature for the membrane) enters through inlet 7 (see FIGS. 1 and 4) and exits the actuator body 2 a through inlet porthole 5 in the domed membrane seat 25 such that the liquid impinges on a hot SMA membrane 3. The heated liquid exits the pump chamber 9 through exit portholes 6 and exits the actuator body 2 a through an outlet 10 (see FIG. 4). As mentioned previously, the outlet 10 is connected to a check valve such that the liquid flows in a single direction out of the actuator body 2 a. This fluid flow configuration over the membrane provides effective convective cooling of the membrane to a temperature below the martensite-austenite transition temperature at which point the bias pressure is again used to move the membrane from its undistorted form to a distorted form.

When charge is applied to the SMA membrane 3 to provide heating to a temperature above the martensite-austenite transition temperature, the deformation of membrane 3 decreases such that the radius of curvature 11 decreases (see FIG. 3). The radius of curvature 11 decreases with a force that is enough to overcome bias pressure applied in the pumping chamber 2 a. The decrease in radius of curvature 11 also increases internal pressure within the pumping chamber 2 a and forces liquid to outlet 10. The liquid is forced out of the check valve, which is adjoined to outlet 10 and exerts pressure to operate a piston located in a cylinder or other actuation device or simply to pump the liquid from one location to another. The internal pressure within the pump chamber 9 decreases as liquid leaves the pump chamber 9. When charge is not applied to the SMA membrane 3, the bias pressure forces cool liquid to enter the pumping chamber through inlet 7. The bias pressure causes the radius of curvature 11 of the cooled membrane 3 to increase.

FIG. 5 illustrates an embodiment with multiple membranes used in parallel to increase the volume flow rate of the system. Consider a cube or box shaped pumping chamber having six sides. The present invention contemplates any number of membranes from 1 to six to be used with the present invention. FIG. 5 illustrates a four-membrane configuration. Charge is applied to the membranes to heat them simultaneously or alternately to multiply the pumping capacity of the device. As shown in FIG. 5, four SMA membranes 3 are assembled at each domed membrane seat face of the actuator body 2 b to allow parallel pumping of liquid. A check valve adjoins inlet 7 such that liquid flows in a single direction into the actuator body 2 b. Cool liquid enters the actuator body 2 b through inlet 7 and simultaneously enters four pump chambers 9 through four inlet portholes 5 such that liquid impinges on hot the four SMA membranes 3. Heated liquid leaves the individual pump chambers 9 through outlet portholes 6. Liquid leaves the actuator body 2 b through outlet 10 which is connected to the portholes 6 by way of an appropriate outlet manifold configuration (not shown). A check valve adjoins outlet 10 such that liquid flows in a single direction out of the actuator body 2 b.

FIG. 6 is a perspective view of a membrane for use in one embodiment of the invention. In the example shown, the membrane has a thickness of 5 micrometers and dimensions of 17 mm wide and 17 mm long. The membrane need not be square but may be any suitable shape without departing from the scope of the invention. The diameter of the circular area where the bias load is applied to the membrane is approximately 11 mm. A current load of approximately 21 amps is used to heat the membrane. The timing of the pump cycle is 1 cycle @ 100 Hz which is equal to a pump cycle of 0.01 seconds. At this timing cycle (100 Hz), the heating time can vary from 1-10% (heating time=0.0001 to 0.001 second and cooling time=0.9999 to 0.999 seconds).

Any of the known SMA materials may be used as the membrane material. NiTi alloys are preferred. In a preferred embodiment of the invention, the composition of the membrane is approximately 53% Titanium and approximately 47% Nickel.

Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the above preferred embodiments and examples, but is only limited by the following claims.

BIBLIOGRAPHY

1. P. Kruelevitch, A. Lee, P. Ramsey, and M. Northrup, “Thin Film Shape Memory Alloy Microactuators”, Journal of Microelectromechanical Systems, 5, no. 4, pp. 270-282, 1996.

2. D. D. Shin and O P. Carman, “Operating Frequency of Thin Film NiTi in Fluid Media,” Proceedings of ASME international Mechanical Engineering Congress and Exposition, 2, New York, Nov. 11-16, 2001.

3. W. L. Benard, H. Kahn, A. H. Heuer, and M. A, Huff, “Thin-film shape-memory alloy actuated micro pumps,” Journal of Microelectromechanical Systems, 7, no. 2, pp. 245-251, June 1998.

4. W. L. Benard, H. Kahn, A. H. Heuer, and M. A. Huff, “A titanium-nickel shape-memory alloy actuated micropumps,” Transducers, International Conference on Solid-State Sensors and Actuators, Chicago, pp. 361-367, Jun. 16-19, 1997.

5. H. Kahn, M A. Huff, and A. H. Heuer, “The TiNi shape-memory alloy and its applications for MEMS,” Journal of Micromechanics and Microengineering, 8, pp. 213-221, 1998.

6. H. Kahn, W. L. Benard, M. A. Huff, and A. H. Heuer, “Titanium-nickel shape memory thin film actuators for micromachined valves,” Materials Research Society Symposium Proceedings, 444, pp. 227-232, 1997.

7. F. Makino, T. Mitsuya, and T. Shibata, “Dynamic actuation properties of TiNi shape memory diaphragm,” Sensors and Actuators, 79, pp. 128-135, 2000.

8. F. Maldno, T. Mitsuya, and T. Shibata, “Micromachining of TiNi shape memory thin film for fabrication of micropump,” Sensors and Actuators, 79, pp. 251-259, 2000.

9. F. Makino, T. Mitsuya, and T. Shibata, “Fabrication of TiNi shape memory micropump,” Sensors and Actuators, 88, pp. 256-262, 2001,

10. F. Makino, K. Kato, and T. Shibata, “Thermo-mechanical properties of TiNi shape memory thin film formed by flash evaporation,” Sensors and Actuators, A Physical, 75, pp. 156-161, 1999.

11. D. Xu, B. Cai, 0. Ding, Y. Zhou, A. Yu, L. Wang, Z. xiaolin, “A novel micropump actuated by thin-film shape-memory alloy,” Proceedings of SPIE, Electronics and Structures for MEMS, 3891, October 27-29, Royal Pines Resort, Queensland, Australia, 1999.

12. D. Xu, L. Wang, 0. Ding, Y. Zhou, A. Yu, B. Cai, “Characteristics and fabrication of NiTi/Si diaphragm micropump,” Sensors and Actuators A, 93, pp. 87-92, 2001.

13. L. Wang, D. Xu, B. Cai, X. Cheng, “Control of internal stress in SMA/Si bimorph microactuators,” Proceedings of SPIE, Micromachining and Microfabrication Process Technology VI, 4174, pp. 340-345, September 18-20, Santa Clara, 2000.

14. D. Xu, L. Wang, 0. Ding, Y. Zhou, A. Yu, X. Cheng, C. Jian, B. Cai, “Dynamic actuation behavior of NiTi/Si diaphragm micropump,” Proceedings of SPIE, Micromachining and Microfabrication Process Technology Vf, 4174, pp. 324-330, September 18-20, Santa Clara, 2000. 

1. An actuator mechanism comprising: an actuator body comprising an actuation chamber having a membrane seat surface; a membrane comprising a shape memory alloy that has a martensite-austenite transition temperature, said membrane being located over said membrane seat surface to define a pump chamber between said membrane seat surface and said membrane, said membrane being movable from an undistorted form to a distorted form; at least one inlet through which fluid is introduced into said pump chamber; at least one outlet through which fluid is removed from said pump chamber, said outlet being located at a spaced location from said inlet; a bias pressure applicator that introduces said fluid into said pump chamber at a temperature that is below said martensite-austenite transition temperature; and a heating system that heats said membrane to an actuation temperature that is above said martensite-austenite transition temperature when said membrane is in said distorted form.
 2. An actuator mechanism according to claim 1 comprising at least two outlets through which fluid is removed from said pump chamber.
 3. An actuator mechanism according to claim 1 wherein said inlet is located centrally in said membrane seat surface and said outlet is located towards a perimeter region of said membrane seat surface.
 4. An actuator mechanism according to claim 2 wherein said inlet is located centrally in said membrane seat surface and said outlets are located towards a perimeter region of said membrane seat surface.
 5. An actuator mechanism according to claim 4 wherein said membrane seat surface has a circular perimeter.
 6. An actuator mechanism according to claim 1 wherein said membrane seat surface is in the form of a dome that extends inwardly into said actuation chamber.
 7. An actuator mechanism according to claim 6 wherein said dome has a center and said inlet is located in the center of said dome.
 8. An actuator mechanism according to claim 7 wherein said dome has a perimeter, and said outlet is located towards the perimeter of said dome.
 9. An actuator mechanism according to claim 6 wherein said dome has a perimeter, and at least two outlets are located in said membrane seat surface and located towards the perimeter of said dome.
 10. An actuator mechanism according to claim 9 wherein said outlets are spaced equidistantly around the perimeter of said dome.
 11. An actuator mechanism according to claim 1 wherein said heating system includes a system for applying an electrical current to said membrane to provide heating thereof.
 12. An actuator mechanism according to claim 30 wherein said inlet flow control mechanism comprises an inlet pressure check valve that prevents flow of fluid from said pump chamber out through said inlet when said fluid in said pump chamber is under a pumping force exerted by said membrane.
 13. An actuator mechanism according to claim 31 wherein said outlet flow control mechanism comprises an outlet pressure check valve that prevents flow of fluid from said outlet back into said pump chamber wherein said fluid in said pump chamber is under a bias force exerted by said bias pressure applicator.
 14. A method for pumping fluid comprising the steps of: A) providing an actuator mechanism comprising: an actuator body comprising an actuation chamber having a membrane seat surface; a membrane comprising a shape memory alloy that has a martensite-austenite transition temperature, said membrane being located over said membrane seat surface to define a pump chamber between said membrane seat surface and said membrane, said membrane being movable from an undistorted form to a distorted form; at least one inlet through which the fluid to be pumped is introduced into said pump chamber; at least one outlet through which said fluid is removed from said pump chamber, said outlet being located at a spaced location from said inlet; B) introducing said fluid into said pump chamber at a temperature that is below said martensite-austenite transition temperature and at a bias force, and C) heating said distorted membrane to an actuation temperature that is above said martensite-austenite transition temperature to exert a pumping force against the fluid in said fluid chamber.
 15. A method for pumping fluid according to claim 14 wherein at least two outlets are located in said membrane seat surface.
 16. A method for pumping fluid according to claim 14 wherein said inlet is located centrally in said membrane seat surface and said outlet is located towards a perimeter region of said membrane seat surface.
 17. (canceled)
 18. (canceled)
 19. A method for pumping fluid according to claim 14 wherein said membrane seat surface is in the form of a dome that extends inwardly into said actuation chamber.
 20. (canceled)
 21. (canceled)
 22. A method for pumping fluid according to claim 19 wherein said dome has a center and a perimeter, and said inlet is located in said center, and at least two outlets are located in said membrane seat surface and located towards the perimeter of said dome.
 23. (canceled)
 24. A method for pumping fluid according to claim 14 wherein said heating of said distorted membrane is accomplished by passing an electrical current through said membrane.
 25. In a method for pumping a fluid wherein a membrane comprising a shape memory alloy is repeatedly heated and cooled, the improvement comprising forced convective cooling of said membrane by introducing a pressurized flow of said fluid into contact with said membrane at an inlet location and flowing said liquid over said membrane to an outlet location that is spaced from said inlet location.
 26. An improved method for pumping fluid according to claim 25 wherein said fluid is flowed from said inlet location over said membrane to at least two outlet locations.
 27. An improved method for pumping fluid according to claim 26 wherein said inlet location is located at the center of the membrane and the outlet locations are located around the perimeter of the membrane.
 28. An improved method for pumping fluid according to claim 27 wherein said outlet locations are located equidistantly around the perimeter of said membrane.
 29. An actuator mechanism according to claim 1 wherein said actuator body includes at least two actuation chambers.
 30. An actuator mechanism according to claim 1, wherein the bias pressure applicator introduces said fluid into said pump chamber at a bias force that is sufficient to move said membrane from the undistorted form to the distorted form.
 31. An actuator mechanism according to claim 30, wherein said membrane at said actuation temperature exerts a pumping force against the fluid in said fluid chamber that is greater than the bias force applied to said fluid by said bias pressure actuator to thereby move fluid out of said pump chamber through said outlet.
 32. A method for pumping fluid according to claim 14, wherein said step of introducing said fluid into said pump chamber comprises introducing said fluid into said pump chamber at a bias force that is sufficient to move said membrane from the undistorted form to the distorted form.
 33. A method for pumping fluid according to claim 32, wherein the pumping force exerted by the membrane is greater than the bias force. 